Gasification of biomass and subsequent combustion to generate power is an interesting small scale system for combined heat and power supply.
Combustion of biomass to produce steam for use in a steam turbine is only effective in large scale systems having an effect for more than 50 MW. System having an effect less than about 10 MW, such as small scale system for combined heat and power supply, are an interesting alternative for industries having need of heat and power and further having an energy rich biomass waste, such as the paper pulp industry. Further, such small scale system for combined heat and power supply are of interest for urban districts having a net for district heating.
There are a lot of different proposals on how gasifier-engine systems may be designed. One of the simplest ways to construct such a system is to use a common fixed-bed gasifier and a standard Otto engine. This solution has for example been used by the American company Community Power Corporation which is producing small scale combined heat and power systems. The British company Biomass Engineering Ltd uses more or less the same technique and has an electric efficiency of about 25% in their 250 kWe pilot plant, where the gasifier had an efficiency of 80% (DTI, Development of a 250 kWe Downdraft Gasifier for CHP. 2006, Biomass Enginering Ltd.).
The BTG Biomass Technology Group in the Netherlands used a downdraft gasifier and had a relatively high engine efficiency of 37% (gas to electricity) at their tests of a 215 kWe plant. At the same time the gasifier efficiency was only 71%, which gave an overall electric efficiency of 27% gross. In this study the temperature of the producer gas was kept over the dew point to avoid production of condensate.
Downdraft gasifiers provide a gas suitable for subsequent combustion. However, a significant part of the carbon initially present in the biomass, typically 4 to 7%, will end up in the residual ash, thus lowering the overall efficiency of the systems comprising a downdraft gasifier. In addition, downdraft gasifiers operate with a rather high oxygen:biomass ratio. Thus, a fairly high degree of the biomass is oxidized already at the gasification stage, lowering the heat value of the generated gas to be combusted subsequently.
A way to get more control over the gasification is to split the process into two stages. Wang et al. used an updraft gasifier combined with a subsequent reformer to crack tars using partial oxidation (Wang, Y., et al., Performance optimization of two-staged gasification system for woody biomass. Fuel Processing Technology, 2007. 88: p. 243-250). The electric efficiency of this system was claimed to be 27% gross, were gross means excluding parasitic losses. The engine they were using was a diesel engine, and to ignite the mixture of air and producer gas diesel oil was used. This means that their system needs both wood chips and diesel oil to operate.
The Biomass Gasification Group in Denmark has designed a two-stage gasifier called Viking (Henriksena, U., et al., The design, construction and operation of a 75 kW two-stage gasifier. Energy, 2006. 31: p. 1542-1553). Here heat from the engine's exhaust gases were transferred to the incoming wood chips in a pyrolysis reactor, where the temperature reached about 600° C. The feed of pyrolysis products, that is gas, vaporized tars and charcoal, enters a downdraft gasifier, operating as a reformer. In the upper part of the gasifier, the tars were partially oxidized with air and reached a temperature of about 1200° C. In the lower part of the gasifier the charcoal was gasified. The fuel to gas efficiency of a 70 kWin pilot plant was 93.2%, the gas to electricity efficiency was 29.1% and the overall fuel to net electricity was 25.1% (Ahrenfeldt, J., et al., Validation of a Continues Combined Heat and Power (CHP) Operation of a Two-Stage Biomass Gasifier. Energy & Fuels, 2006. 20: p. 2672-2680). The efficiency of their engine was not as impressive as the gasifier and this was partly because no supercharging was used and that the engine only operated at part load.
Systems comprising a reformer suffer from the need for a rather high oxygen:biomass ratio in the gasification step(s). Thus, a fairly high degree of the biomass is oxidized already at the gasification stage, lowering the heat value of the generated gas to be combusted subsequently.
The residence time of biomass in systems of the art is fairly long. As a consequence, systems in the art all suffer from demanding long periods of time for start up and equilibration. From a safety perspective, i.e. to lower the risk of explosions and conflagration, a lower amount of biomass present in the system would also be desirable.
Further, systems in the art are adopted solely for solid biomass and are hence not suitable for gasification of alternative streams of carbonaceous liquid. In addition, systems in the art are not suitable for varying the fed of biomass over time. Furthermore, the above mentioned systems are all fairly complex and hence costly.
Thus, there is need within the art for an alternative process and a system for gasifying biomass to obtain synthesis gas mitigating, alleviating, eliminating, overcoming or circumventing the above mentioned problems.